为什么java并发测试失败？

Question

我有一个简单的类：

import org.slf4j.Logger; 
import org.slf4j.LoggerFactory; 

public class DummyService { 
    private final Logger logger = LoggerFactory.getLogger(getClass()); 
    private boolean dataIndexing = false; 

    public boolean isDataIndexing() { 
     logger.info("isDataIndexing: {}", dataIndexing); 
     return dataIndexing; 
    } 

    public void cancelIndexing() { 
     logger.info("cancelIndexing: {}", dataIndexing); 
     dataIndexing = false; 
    } 

    public void createIndexCorp() { 
     logger.info("createIndexCorp: {}", dataIndexing); 
     createIndex(); 
    } 

    public void createIndexEntr() { 
     logger.info("createIndexEntr: {}", dataIndexing); 
     createIndex(); 
    } 

    private void createIndex() { 
     logger.info("createIndex: {}", dataIndexing); 
     if(dataIndexing) 
      throw new IllegalStateException("Service is busy!"); 
     dataIndexing = true; 
     try { 
      while(dataIndexing) { 
       Thread.sleep(100); 
       logger.debug("I am busy..."); 
      } 
      logger.info("Indexing canceled"); 
     } catch (InterruptedException e) { 
      logger.error("Error during sleeping", e); 
     } finally { 
      dataIndexing = false; 
     } 
    } 
} 

和一个单元测试，与我想测试对象行为：

public class CommonUnitTest 
{ 
    @Test 
    public void testCreateIndexWithoutAsync() throws InterruptedException { 
     final long sleepMillis = 500; 
     final DummyService indexService = new DummyService(); 
     assertFalse(indexService.isDataIndexing()); 
     new Thread(() -> { 
        indexService.createIndexCorp(); 
       } 
     ).start(); 
     Thread.sleep(sleepMillis); 
     assertTrue(indexService.isDataIndexing()); 
     // TaskExecutor should fails here 
     new Thread(() -> { 
        indexService.createIndexEntr(); 
        logger.error("Exception expected but not occurred"); 
       } 
     ).start(); 
     assertTrue(indexService.isDataIndexing()); 
     indexService.cancelIndexing(); 
     Thread.sleep(sleepMillis); 
     assertFalse(indexService.isDataIndexing()); 
    } 
} 

对象的行为必须是：如果该方法createIndexCorp或createIndexEntr是由一个线程调用，那么另一个线程必须通过尝试调用其中一个方法来获得异常。但是这不会发生！这里是日志：

2015-10-15 17:15:06.277 INFO --- [   main] c.c.o.test.DummyService     : isDataIndexing: false 
2015-10-15 17:15:06.318 INFO --- [  Thread-0] c.c.o.test.DummyService     : createIndexCorp: false 
2015-10-15 17:15:06.319 INFO --- [  Thread-0] c.c.o.test.DummyService     : createIndex: false 
2015-10-15 17:15:06.419 DEBUG --- [  Thread-0] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:06.524 DEBUG --- [  Thread-0] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:06.624 DEBUG --- [  Thread-0] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:06.724 DEBUG --- [  Thread-0] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:06.818 INFO --- [   main] c.c.o.test.DummyService     : isDataIndexing: true 
2015-10-15 17:15:06.820 INFO --- [   main] c.c.o.test.DummyService     : isDataIndexing: true 
2015-10-15 17:15:06.820 INFO --- [  Thread-1] c.c.o.test.DummyService     : createIndexEntr: true 
2015-10-15 17:15:06.820 INFO --- [   main] c.c.o.test.DummyService     : cancelIndexing: true 
2015-10-15 17:15:06.820 INFO --- [  Thread-1] c.c.o.test.DummyService     : createIndex: true 
2015-10-15 17:15:06.824 DEBUG --- [  Thread-0] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:06.921 DEBUG --- [  Thread-1] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:06.924 DEBUG --- [  Thread-0] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:07.021 DEBUG --- [  Thread-1] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:07.024 DEBUG --- [  Thread-0] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:07.121 DEBUG --- [  Thread-1] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:07.124 DEBUG --- [  Thread-0] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:07.221 DEBUG --- [  Thread-1] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:07.224 DEBUG --- [  Thread-0] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:07.321 DEBUG --- [  Thread-1] c.c.o.test.DummyService     : I am busy... 
2015-10-15 17:15:07.321 INFO --- [   main] c.c.o.test.DummyService     : isDataIndexing: true 

你可以看到第二个线程可以启动进程，但它应该会得到异常。测试代码中的最后一个断言也失败了。这怎么可能发生？我不明白这种行为。我试图使用volatile和synchronized关键字，但没有任何帮助。 DummyService有什么问题？

Answer 1

你有3个线程，T0，T1和TM（主）。 操作的顺序是这样的：

tm starts t0 
t0 checks dataIndexing flag - false, goes into the loop, sets flag to true 
tm sleeps 
tm starts t1 
tm sets indexing flag to false 
t1 checks dataIndexing flag - false, goes into the loop, sets flag to true 
t0 continues the loop because it missed that brief period when indexing was cancelled 

如果您在主索引TM标志设置为false之前睡觉，那么T1会得到异常。 您需要同步对多个线程之间共享的变量的访问。即检查标志的状态并且改变它需要在持有互斥体时完成。

Answer 2

看来你打的记录和实际执行之间的差异。线程可以想象地运行取消并在日志和异常之间的空间中创建索引，从而第二个线程滑倒并防止取消第一个和第二个线程。

这是不可取的，允许共享资源，即private boolean dataIndexing同时变化。有两个解决方案（至少）：

1.A syncronized方法，以允许所述共享资源的改变（因此限制只访问一个线程时间）

private synchronized void setDataIndexing(boolean value) { 
    dataIndexing = value; 
} 

2.Guarding每个改变在syncronized部分（以及= true两个= false地）这个值：

syncronized (this) { 
    dataIndexing = /* the relevant value */; 
} 

我会建议一个单独的方法，但良好的知道的替代品。

Answer 3

不回答你的问题，但是这是完全不同步：

if (dataIndexing) 
    throw new IllegalStateException("Service is busy!"); 
dataIndexing = true; 

是业务繁忙，如果你的执行到达throw声明？不必要！另一个线程可能会在测试和抛出之间将dataIndexing的值从true更改为false。

更糟的是，也许差多了，是两个线程可能都扔在同一时间后到达的声明：

Thread A       Thread B 

tests dataIndexing, finds it to 
be false. 

            Tests dataIndexing, finds it to be false. 

sets dataIndexing = true;   sets dataIndexing = true; 
...        ... 

---

此外，这是不可靠的，这需要时间。

Thread.sleep(sleepMillis); 
assertTrue(indexService.isDataIndexing()); 

更好地设计你的类可测性。如果你的测试需要等到isDataIndexing()，那么你的班级应该提供一个测试手段到wait() ......

另外，不要低估测试在尽可能少的时间内完成的重要性。当你有一个拥有数千或数万测试用例的系统时，秒数真的开始累加起来。